METHOD OF MAKING A PRINTED CIRCUIT BOARD

Abstract: A method of making a plate through hole printed circuit board, comprises the steps of: a) forming conductive circuit elements (10) on two opposed faces of a paper phenolic containing substrate (11); b) coating the substrate and circuit elements with a de-sensitising material (12); c) forming holes (13) through the substrate, each hole passing through a circuit element on each of the opposed faces of the board; d) treating the board to render the substrate exposed in the holes receptive to the action of a metallic plating solution; e) removing the de-sensitising material; f) printing an acid-resist mask (14) on to both faces of the board, the mask leaving exposed only a small area of the board surrounding each hole therethrough; and g) treating the board with an electroless nickel-plating solution to deposit nickel (15) in the holes to the desired thickness to provide an electrical connection through each hole between two opposed conductive circuit elements.

Description: METHOD OF MAKING A PRINTED CIRCUIT BOARD Field of the Invention This invention relates to a method of making a printed circuit board of the plate through hole type, and in particular to a method using board of the paper phenolic material type. Plate through hole (PTH) boards have conductive circuit elements formed on both sides and interconnected by holes drilled through the board from a con¬ ductive element on one side to an opposed element on the other side, the holes having conductive metal plated therethrough. Background to the Invention In my earlier published application W093/26145, I disclosed and claimed a method of making PTH boards using electroless plating. While the general method is suitable for use on a wide range of substrates, it has not been possible to use the most widely-used substrate, laminated paper bonded with phenolic resins, referred to as "paper phenolic materials", because prolonged exposure to the hot al¬ kaline copper plating solutions required causes breakdown of the substrate, and be¬ cause such materials contain traces of antimony, which can poison a copper-plating bath and render it ineffective. Summary of the Invention The present invention provides a method of making a plate through hole printed circuit board, comprising the steps of: a) forming conductive circuit elements on two opposed faces of a paper phenolic containing substrate; b) coating the substrate and circuit elements with a de-sensitising material; c) forming holes through the substrate, each hole passing through a cir¬ cuit element on each of the opposed faces of the board; d) treating the board to render the substrate exposed in the holes recep¬ tive to the action of a metallic plating solution; e) removing the de-sensitising material; f) printing an acid-resist mask on to both faces of the board, the mask leaving exposed only a small area of the board surrounding each hole there¬ through; and g) treating the board with an electroless nickel-plating solution to deposit nickel in the holes to the desired thickness to provide an electrical connection through each hole between two opposed conductive circuit elements. Preferably, the de-sensitising material is a plating resist coating material. The metallic plating solution is preferably a non-homogeneous plating solution, which will only plate on to areas sensitised or treated to render them receptive to the plating. Such sensitising typically deposits a catalyst on to a cleaned and micro-etched surface to initiate the deposition of metal from its solution. In comparison with conventionally produced boards, boards produced by the method of the invention are consistent in quality, and in uniformity of plating through the holes. Since metal is not built up equally over the whole board dur¬ ing plating, but is concentrated on the holes, the initial metal coating on the sub- strate can be selected to be the design conductor thickness, thus ensuring that all tracks meet the design specification. This is very important in controlling the im¬ pedance of high frequency circuits. The use of a single electroless plating operation before imaging ensures that no voids are present which might give rise to out- gassing, and contamination cannot occur. Further, although plating in the holes may not proceed uniformly initially, because of the variation in the surface smoothness in the drilled, or preferably punched hole, the resultant lack of smoothness in the surface of the plated metal through the hole is advantageous in providing a key for the solder in the subsequent use of the board. The imaging of the conductive tracks is carried out before the holes are drilled or punched, and involves the formation of a positive resist image. This means that screen printing of an alkali-strippable resist can be used. This is not only a very low-cost operation, but requires only treatment in an alkaline solution to remove the resist, preferably the alkaline accelerator bath used after sensitising. Such treatment does not give rise to difficulties in waste treatment, requiring no organic solvents. The method of the invention uses only one plating bath, reducing cost and the problems associated with treatment of waste from the plating stage. Capital costs and running costs are substantially smaller than with the conventional proc¬ ess. Control of the plating process is simple and a high degree of precision in achieving design plating thicknesses can be expected. Benefits arise from the use of electroless nickel plating solutions to deposit conductive metal in the holes. Firstly, plating rates are very much higher than for copper solutions, typically around 25 μm/hr, so that a satisfactory thickness of metal can be built up in a much shorter time. Secondly, since nickel does not tar¬ nish in the way that copper does, it is no longer necessary to coat the exposed cop¬ per on the board with solder. This is conventionally done in a hot-air levelling process, involving dipping the board into a bath of solder and then removing ex- cess solder and levelling by means of a jet of hot air. This process is costly and po¬ tentially dangerous, and can give rise to health risks due to metal vapour emissions. Thirdly, electroless nickel plating does not require the presence of the formaldehyde required for copper plating. Since formaldehyde is carcinogenic, its avoidance is very desirable. Boards plated through the holes with nickel in accor- dance with the invention have been found to meet the highest quality standards, rendering them suitable for use in space and aerospace applications, while produc¬ tion costs are comparable with conventional standard production boards. The acid plating bath used in the electroless deposition of nickel does not attack paper phenolic materials, thus opening up such materials to high quality electroless through-hole plating. Brief Description of the Drawings In the drawings: Figures 1 to 4 are diagrammatic sectional views through a board illustrating stages in a manufacturing process according to the present invention. Detailed Description of the Illustrated Method of the Invention Referring to Figure 1 , the method of the invention comprises as a first step the formation of the conductive circuit elements 10 on each surface of the sub¬ strate 11 by the conventional processes of forming a positive resist image, for exam¬ ple by silk screen printing, or photographically, and etching away the copper from the exposed parts of the board. A uniform coating 12 of an alkali-soluble plating resist is then applied to each surface of the board by silk screen printing or dry film application as a de-sensitising layer, and is cured, for example by the application of U.V. light. Holes 13 are then punched through the board and the de-sensitising layer at the desired locations for interconnections between opposed circuit elements 10, yielding a configuration as shown in Figure 2. Treatment of the exposed substrate in the holes to render it receptive to electroless plating of nickel is then carried out in conventional manner, with initial cleaning, micro-etching and sensitising with any of the known electroless preparatory systems, for example the materials sold by MacDermid GB Ltd under reference 9027. The next stage in preparation is treat¬ ment by caustic soda as an accelerator, and this also has the effect of stripping off the de-sensitising resist layer 12. A mask 14 of acid resist is then printed on each face of the board, leaving only a small area (approximately 2mm wide) around each hole exposed, as shown in Figure 3. Since the only area of the board prepared for plating of nickel is the exposed surface within and around the holes 13, treatment of the board in a strong electroless nickel plating solution, e.g. those sold by MacDermid GB Ltd under ref¬ erences ELNIC 105 or ELNIC 110, for approximately one hour, builds up nickel to the desired thickness in and around the holes only. This leaves the configuration illustrated in Figure 4. It is observed that the thickness of nickel 15 through the holes is generally uniform, and consistent across the area of the board, the plating of nickel being less subject to the faults which can arise in the conventional elec¬ troplating techniques. Control of the electroless plating stage is conveniently achieved as follows. The boards are suspended in a tank of the plating solution, which is suitably circu¬ lated through an overflow tank, where it can be filtered, so as to maintain a con¬ stant volume in the plating tank. Air is bubbled through the plating tank to agitate the solution and ensure adequate mixing, and the supports for the boards are oscillated to ensure that fresh solution is constantly passed through the holes. The plating solution is replenished from a measured supply by a dosing pump or pumps, which operate at a constant predetermined rate, but which are controlled to switch on and off repeatedly so as to run for a set percentage of the total plating time. This is determined according to the amount of nickel to be deposited.

Claims: CLAIMS 1. A method of making a plate through hole printed circuit board, com¬ prising the steps of: a) forming conductive circuit elements on two opposed faces of a paper phenolic containing substrate; b) coating the substrate and circuit elements with a de-sensitising material; c) forming holes through the substrate, each hole passing through a cir¬ cuit element on each of the opposed faces of the board; d) treating the board to render the substrate exposed in the holes recep¬ tive to the action of a metallic plating solution; e) removing the de-sensitising material; f) printing an acid-resist mask on to both faces of the board, the mask leaving exposed only a small area of the board surrounding each hole there- through; and g) treating the board with an electroless nickel-plating solution to deposit nickel in the holes to the desired thickness to provide an electrical connection through each hole between two opposed conductive circuit elements. 2. A method according to Claim 1, wherein the de-sensitising material is a plating resist coating material. 3. A method according to Claim 1 or 2, wherein the plating solution is a non-homogeneous plating solution. 4. A method according to any preceding claim, wherein step (d) comprises deposit of a catalytic material on the exposed substrate. 5. A method according to any preceding claim, wherein the holes are formed by punching.

Inventors: KNOPP JOHN FREDERICK DAVID, KNOPP, JOHN, FREDERICK, DAVID
Applicants: KNOPP JOHN FREDERICK DAVID, KNOPP, JOHN, FREDERICK, DAVID
Application year: 1994
Main classification: H05K3
IPC codes: H05K3